1. Field of the Invention
The present invention relates to ball screws of the type having internal return means, and more particularly to an externally servicable ball screw having internal return means.
2. Description of the Prior Art
The advantages of mechanical ball screws as converters of rotary to linear motion with extreme accuracy and low friction has been known in the art for many years. The first type of ball screw to be developed was one having an external return tube whereby the balls interposed between the ball screw and the ball nut travelled outside the actual ball nut to make their return from trailing edge of the ball nut back to the leading edge of the nut for continuous recirculation. It was found, however, that becuase of the additional space required by the external return tubes, many applications could not adapt to the use of such ball screws. Therefore, a ball screw having an internal return means was developed which satisfactorily fit in many smaller spaces. However, it was found that both type of these ball screws have a serious problem as they age and become worn. As the balls and/or ball races become worn, a looseness of fit develops and the ball screws lose their accuracy, although they retain their low friction.
Due to the expense of the ball screw, it has been found desirable to rebuild ball screws to the extent possible, rather than replacing them. Such rebuilding can involve something as simple as placing slightly larger ball bearings in the ball races, to re-turning the ball screw and/or ball nut.
Whichever method of rebuilding is used, a serious problem exists in reassembling the ball screw after it has been disassembled for rebuilding. This involves holding the balls in the races of the ball nut while screwing the ball screw into position. This becomes a problem because, whether the external or internal return means are used, balls cannot be placed into the ball race of the ball nut from the outside of the ball screw assembly. This means that the tracks, must be greased, the balls must be placed in the tracks, and the ball screw carefully rotated in the ball nut to prevent the balls from falling out. It can be understood that this reassembly can be a rather long and laborious process, and may cost more than the actual rebuilding of the ball screw.